A variety of optical frequency synthesizers are known. However, these synthesizers suffer from poor efficiency, coarse spacing of multi-frequency combs, poor resolution, instability and other issues. While there is ever growing and urgent need for programmable, very high frequency (hundreds of GHz) and precise optical references, the prior art techniques are not suitable to synthesize these tones. For example, a single-frequency tunable laser offset locked by means of an analog phase-locking loop to a fixed-frequency reference laser can be considered an optical frequency (OF) synthesizer whose offset frequency (between these two lasers) is determined by an RF source. One possible RF source is an amplified electronic direct digital synthesizer (DDS) circuit coupled to a digital-to-analog converter (DAC) or DDS/DAC circuit. As the frequency of the RF source is varied, the OF output of the tunable laser likewise is varied. However, the span of frequencies that this OF synthesizer can produce is limited to the span of frequencies produced by the RF source, which is typically only several GHz for an electronic DDS/DAC.
Another example is a laser comb generator that can be provided by a mode-locked laser that produces a train of optical pulses or by coupling higher power CW laser light into an optical micro-resonator that produces a comb by means of cascaded four-wave mixing processes. Such a comb generator can be considered an OF synthesizer whose output frequency is obtained by selecting a line of the comb. However, the number of comb lines produced by such comb generator is limited. Typical chip-scale mode-locked lasers produce a comb of 10 to 100 lines, with 1000 lines being produced in extreme cases. Comb generators based on four-wave mixing in a micro-resonator likewise produce combs having at most several thousand lines (or tones). The combs produced by these comb generators can have span of frequency values exceeding an octave in some cases. However, the increment in the possible frequency values of such a synthesizer with a fixed-spacing comb is quite coarse, typically 10 GHz or larger for a compact comb generator of chip-scale size.
Hence there is an urgent need in the field of optical frequency synthesizers for a synthesizer that is highly efficient, compact, programmable and able to generate high frequency optical references with great precision and stability.